Hello!
As the theme[1] for Parkes is astronomy, and because Parkes is a
observatory[2], I propose the login sound to be inspired from the
sound
of some real pulsars recorded. Look here the sounds of the pulsars
in
47 Tucanae[3](very interesting):
http://www.jb.man.ac.uk/~pulsar/Education/Sounds/47tuc1-8000.wav
or here
http://www.jb.man.ac.uk/~pulsar/Education/Sounds/47tuc2-8000.wav
The problem is I couldn't find sounds released under the creative
commons license.
[1] - http://www.gnewsense.org/ArtworkProjects/Parkes
[2] -
https://secure.wikimedia.org/wikipedia/en/wiki/Parkes_Observatory
[3] - more pulsar sounds -
http://www.jb.man.ac.uk/~pulsar/Education/Sounds/